The invention is generally related to shaped charges having reduced slug creation.
After a well has been drilled and casing has been cemented in the well, perforations are created to allow communication of fluids between pay zones in the formation and the wellbore. Shaped charge perforating is commonly used, in which shaped charges are mounted in perforating guns that are conveyed into the well on either an electric line (e.g., a wireline) or tubing (e.g. production tubing, drill pipe, or coiled tubing).
Shaped charges are considered "high explosives"; that is, they detonate at very rapid rates and generate tremendous pressures. There are two types of shaped charges: generally conical shaped charges designed of deep penetration into a formation; and substantially non-conical shaped charges designed for creation of big holes through the casing and shallow penetration into the surrounding formation.
Referring to FIG. 1A, a generally conical shaped charge 10 includes an outer case 12 that acts as a containment vessel designed to hold the detonation force of the detonating explosive long enough for a perforating jet to form. Common materials used for the outer case 12 include steel, zinc, aluminum, ceramics, and glass.
A main explosive charge 16 is contained inside the outer case 12 and is sandwiched between the inner wall of the outer case 12 and the outer surface of a liner 20 that has generally a conical shape. A primer 14 provides the detonating link between a detonating cord (not shown) and the main explosive charge 16. The primer 14 is initiated by the detonating cord, which in turn initiates detonation of the main explosive charge 16 to create a detonation wave that sweeps through the shaped charge 10.
Referring to FIG. 1B, upon detonation, the liner 20 (original liner 20 represented with dashed lines) collapses under the detonation force of the main explosive charge 16. Material from the collapsed liner 20 flows along streams (such as those indicated as 29) to form a perforating jet 26 along the X axis. If the liner 20 is constructed of a solid metal, a slug 28 (sometimes referred to as a "carrot") is also formed as a byproduct of the explosive detonation and liner-to-jet formation process. With deep perforations having relatively small hole diameters, these slugs can plug up the perforated tunnels and potentially reduce fluid flow.
Different portions of the liner 20 contribute to creation of the slug 28 and the perforating jet 26. The inner conical portion 30 of the liner 20 forms the jet 26 while the outer conical portion 32 of the conical liner 20 forms the slug 28. For conical liners, the partition between the jet-producing and slug-producing portions of the liner lies along a cone (represented as 31) between the inner and outer portions 30 and 32. A point P represents a point of stagnation that divides the slug 28 and the perforating jet 26 for conical liners. The exact location of the separation surface 31 depends on the apex angle of the conical liner and other factors, but all liners of a generally conical shape exhibit this type of separation between slug-producing and jet-producing liner regions.
To reduce or eliminate formation of these slugs, conical liners formed of powdered metal have been used. The powdered metal does leave behind a mass of non-jet material, but the non-solid material is distributed along the perforated hole and does not form a solid slug.
Conical shaped charges have also used bi-metallic liners to reduce or eliminate formation of the slugs. A bi-metallic liner includes two layers of metal, both conically shaped, that are pressed to fit together to form a first cone (which contacts the main explosive charge) and a second cone (which faces the air side). One layer contributes to formation of the perforating jet, while the other layer, if selected of an appropriate metal such as zinc, disintegrates so that formation of a solid slug is reduced.
The other type of shaped charge, the substantially non-conical shaped (e.g., pseudo-hemispherical, parabolic or other similar shape) charge, is designed to create large entrance holes in casing and reduced penetration into the cement or formation. These types of shaped charges are also referred to as big hole charges. Solid metal liners as well as powdered metal liners have been used with the big hole shaped charges. Use of solid metal liners in these charges can also produce slugs. To reduce or eliminate the slug in the big hole shaped charges, powdered metal liners have been used. However, use of powdered metal liners have typically reduced performance of these charges as well as increase manufacturing complexity.
Another proposed shaped charge uses a wrought copper alloy liner, which includes an alloy that is multiple phase; that is, the alloy includes a ductile matrix and a discrete second phase, the second phase having a melting temperature less than the temperature reached by the liner after detonation.